[FR] Support OpenAI baseurl/proxies in some form

### Description

Currently the openai api points are hardcoded into the code (api.openai.com ...).

This prevents usage over proxies where the normal site may be blocked, or usage with compatible apis such as azure's openai or localai.

I suggest adding a baseurl option along with the openai key option, that will default to the openai endpoint but will let users choose.

### Impact

Users that can't use the normal openai endpoint can still benefit from the AI features in appflowy.

### Additional Context

_No response_